After temporary bike lanes were set up in Berlin, more people started cycling. The researchers wanted to know why. They found that people's beliefs about what others do (social norms) affect how they travel. People are more likely to bike if they see others doing it. This is true even when comparing different cities with different bike lanes. Pictures of bike-friendly infrastructure also influence people's beliefs about biking. Seeing both good infrastructure and lots of cyclists makes people more likely to bike.